Use transition_reveal in gganimate to cumulatively plot lines

Is there a way to use transition_reveal() in gganimate to draw lines one by one - or a workaround using other transition functions..

library(tidyverse)
library(gganimate)
set.seed(1)
d <- tibble(x = 1:100, a = cumsum(rnorm(100)), b = cumsum(rnorm(100))) %>%
  pivot_longer(cols = a:b, names_to = "grp", values_to = "y")
d
# # A tibble: 200 x 3
#        x grp        y
#    <int> <chr>  <dbl>
#  1     1 a     -0.626
#  2     1 b     -0.620
#  3     2 a     -0.443
#  4     2 b     -0.578
#  5     3 a     -1.28 
#  6     3 b     -1.49 
#  7     4 a      0.317
#  8     4 b     -1.33 
#  9     5 a      0.646
# 10     5 b     -1.99 
# # ... with 190 more rows

I would like for only line to be revealed at a time - so that the plotting for the second line commences after the first is complete (on frame 101) - rather than both lines being plotted at the same time...

ggplot(data = d, mapping = aes(x = x, y = y, colour = grp)) +
  geom_line() +
  transition_reveal(along = x)

Solution

We could make a helper column that puts the points for b after all the points for a:

d %>%
  arrange(grp, x) %>%
  mutate(x_reveal = row_number()) %>%
  ggplot(aes(x = x, y = y, colour = grp)) +
    geom_line() +
    transition_reveal(along = x_reveal)